1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which magnetically excites the nuclear spin of an object with a high-frequency (RF) signal having the Larmor frequency and reconstructs an image from a magnetic resonance signal generated along with the excitation.
2. Description of the Related Art
A medical imaging apparatus provides a large amount of information on a patient in the form of an image, and plays an important role in many medical practices including the diagnosis of a disease, the treatment, and the operation planning. Currently, major medical imaging apparatuses include an ultrasonic diagnostic apparatus, an X-ray CT (computerized tomography) apparatus, a magnetic resonance imaging (hereinafter referred to as MRI) apparatus, and a nuclear medicine diagnostic apparatus. In particular, the MRI apparatus can collect a high-quality contrast image of soft tissue, and occupies an important place in the medical imaging diagnosis.
The MRI apparatus is an apparatus which uses gradient coils to generate gradient magnetic fields in the directions of the X-axis, the Y-axis, and the Z-axis in the field of view for imaging a patient set inside a cylindrical static field magnet that generates a static field, and which transmits a high-frequency signal from an RF (radio frequency) coil to magnetically resonates the nuclear spin in the patient and reconstruct an image of the patient with the use of an NMR (nuclear magnetic resonance) signal generated by the excitation.
The MRI apparatuses of recent years include the one using a coil system which increases the moving distance of a table-top carrying thereon the patient and includes a plurality of coils under the table-top to thereby enable the imaging of a wide area.
FIG. 36 is a diagram illustrating a configuration of a conventional MRI apparatus.
A conventional MRI apparatus 70 is formed by a whole body coil 71 as an RF coil provided in the circumferential direction about the body axis of a patient P, an upper coil 72 as an RF coil provided above the patient P, a lower coil 74 as an RF coil provided under a table-top 73, on which the patient P is placed, and moved integrally with the table-top 73 in the direction of the Z-axis, and a bed structure 75 for advancing and retreating the table-top 73 in the direction of the Z-axis with respect to a cavity formed in a gantry.
Further, to improve the S/N (signal to noise) ratio when the region to be imaged is the head or a foot, a special RF coil for imaging a local site is employed as an RF coil having a chassis substantially fitting the outer shape of the region to be imaged so that the signal is received at a position as close to the body surface of the imaged region as possible. For example, a head coil 76 is attached to the head of the patient P, and a foot coil 77 is attached to a foot of the patient P.
According to the MRI apparatus 70, the NMR signal of each of the imaged regions received by the lower coil 74 is received by a receiver 79 via a received signal cable 78, and an image is generated on the basis of the received signal. The image generated by the MRI apparatus 70 is used for cancer screening and so forth.
However, the conventional MRI apparatus including the coil system enabling the imaging of a wide area entails the following disadvantages.
Firstly, in the imaging of the whole body, for example, the moving distance of the lower coil needs to be at least the sum of the height of the patient and the distance from the leading end to the center of the gantry. Thus, the received signal cable connected to the lower coil is increased in length. As a result, the S/N ratio of the signal received by the receiver is deteriorated.
Secondly, the patient needs to be positioned in accordance with the lower coil, and the positioning takes time. Further, the burden on the patient caused by the time taken for the positioning is large.
Thirdly, the coil on the table-top is displaced from the coil position optimized for each of the sites, depending on the physical size of the patient. As a result, good imaging is prevented.
Fourthly, since the coil is provided on the table-top, an extra coil set is required for a stretcher in which the patient is previously positioned on the table-top.
Fifthly, in the use of the special RF coil for imaging a local site, the RF coil is coupled with the coil provided on the table-top. As a result, the performance of the coils cannot be optimized.